This invention relates to an improved plastic drum for clothes washing appliances, in particular washing machines of the household type.
Washing machines, in particular of the front-loading type, are known in which the clothes to be washed, or the washload, are contained in appropriate perforated, i.e. non-watertight, containers that are generally termed as "drums". The drums are manufactured according to various methods and consist essentially of a rigid structure formed by a front wall, a back plate and an outer cylindrical mantle.
The drums are installed in an outer watertight container, generally termed a "tub" containing the washing liquid, and are suspended inside the tub by means of a mounting element with a radial structure, named a spider journal. The spider journal supports the drum through a supporting shaft that doubles as a driving shaft adapted to convey the rotary motion to the drum. The drum is routed through an appropriate sealed aperture provided in the tub.
Furthermore, oil the inner side of the afore cited outer cylindrical mantle of the drum, a number of vane-like elements, named "ribs" are usually provided in order to promote agitation of the washload in the drum.
Such drums are essentially fabricated using two types of manufacturing methods and related materials. The most traditional method consists of a cylindrical metal structure in which the two walls and the cylindrical mantle are made using stainless sheet-steel and appropriate fabrication processes therefor.
While quite reliable, this type of drum involves considerable fabrication and assembly costs since each one of its component parts has to be fabricated separately, with the use of respective dies and equipment, and finally assembled.
A second, more recent type of drum consists of a structure made of plastic material obtained by separately molding two portions of the drum and then joining these two portions together with the so-called "specular" welding technique, or a similar technique. The structure can also be obtained, as disclosed in the European patent application no. 472 840 A2, with a method that comprises a first phase in which, by using an appropriate mold capable of molding not only the structure of the drum, but also the related spider journal and ribs at the same time. The rigid structure of the drum is obtained by injection molding the plastic material into the cavity of the mold in such an amount as to partially fill the portions of the mold cavities defined between a stationary plate and a moving plate of the mold, while the front wall of the drum is molded separately and assembled later on.
In the described method, before the plastic material is injected into the appropriate mold cavities, the drum supporting shaft is introduced on a corresponding projection in the afore cited mold plate.
Such a method has the advantage of a considerable simplification of the manufacturing process and a remarkable saving in material usage.
However, a still incomplete drum, i.e. a drum that shall be further joined to its front wall through an additional assembly operation, is obtained with this method. Furthermore, the drum that is obtained in this way proves to be quite a delicate construction in practice, since the gravity and rotary stresses imparted by the driving shaft are transmitted to the drum structure through a spider journal of plastic that is shrink-fitted directly on the shaft. Due to the notorious fact that plastics are not very tough materials and therefore tend to become loosened quite easily, the plastic spider journal proves generally to be rather fragile and suited only to light-duty applications.
A rotating drum of plastic material for clothes washing machines is also known, as disclosed in the Italian patent application no. 6993/B/85, to be provided on one side with a coaxial supporting and driving shaft supported on the back side, with further supporting means of the rolling bearing type that are mounted between the drum and the tub.
Such an embodiment appears, however, to be quite difficult to be implemented in practice due to the fact that said interposed rolling support means, typically bearings, have the effect of acting as brakes and considerably retarding the rotating motion of the drum at its high spinning speeds. Furthermore, the bearing means are quite likely to become damaged since they are almost constantly submerged in the washing liquid. Thus, their cost and assembly would inevitably put an additional heavy burden on the production economics while at the same time impairing the overall reliability of the tub/drum assembly.
A rotating drum of plastic material for top-loading clothes washing machines is also known, as disclosed in the Italian patent application no. 7048/B/85, to consist of a perforated cylindrical mantle, to which two corresponding head flanges are applied on both sides. The flanges are fastened to the mantle with techniques and methods that are well-known in the art.
Even such an approach has clear drawbacks due to the need of setting up three different molds, producing the three component parts of the drum separately. Then assembling them by means of such mechanical fasteners as rivets, bolts and/or tie-rods or screw-stays, which adds a further operation that makes the production of the drum more complicated and its costs significantly higher.